1. Field of the Invention
The present invention relates to semiconductor device manufacturing technology and particularly to photolithography technology for forming photo resist patterns on semiconductor substrates.
2. Description of the Related Art
Photolithography technology used in the semiconductor device manufacturing process to process films of various materials in shape is well known. A photolithography process comprises process steps similar to those of usual photography. That is, by going through a resist applying step of applying a photo-resist of photosensitive resin on wafers, an exposure step of projecting exposure light through a mask having a planar pattern formed thereon onto a material layer to transfer the pattern of the mask to the photo-resist, and a developing step of forming a transferred resist pattern by photochemical reaction, the resist pattern is formed on the wafers. Using the resist pattern formed in this way as a mask, subsequent etching of a subject layer, ion injection, or the like is performed.
FIGS. 1A to 1F show an example of process steps of a well-area forming process including the conventional photolithography process. The well-area refers to a well-like impurity injected area formed along the surface of a semiconductor substrate to form transistors or the like on the substrate. First, a semiconductor substrate 10 is washed and dried (FIG. 1A). Then, a photo-resist of photosensitive resin is applied uniformly on the semiconductor substrate by, e.g., a spin coat method. Thereafter, heat treatment called prebake is performed (FIG. 1B). Then, the wafer coated with the photo-resist is set in an exposure apparatus, and by projecting exposure light such as ultraviolet through a mask 100, the pattern of the mask is transferred to the photo-resist (FIG. 1C). After overheat treatment called post-exposure bake for promoting chemical reaction in the resist film is performed on the exposed wafer, a strong alkaline developer is sprayed to remove the exposed portions of the photo-resist. After the development, the wafer is washed with dedicated rinse, pure water, or the like, and drying treatment called postbake is performed thereon (FIG. 1D). Next, using the resist pattern formed on the wafer as a mask, impurity ions are injected into the semiconductor substrate 10 to form a well area 12 (FIG. 1E). Subsequently, the photo-resist left on the wafer is removed by ozone ashing, and thereafter a sulfuric acid wash is performed. Going through the above steps, the well-area forming process finishes (FIG. 1F).
In the exposure process step of FIG. 1C, before each exposure by the exposure apparatus, the position in the thickness direction of the semiconductor substrate is measured by a focusing detection system of the exposure apparatus, and through feedback control using the measurement, the stage on which the wafer is mounted is positioned in the substrate-thickness direction to perform exposure. When the position in the thickness direction of the semiconductor substrate is correctly detected and correct feedback control is executed in positioning the wafer stage, a resist pattern having a desired shape and dimensions can be obtained. By the way, in detecting the position in the thickness direction of a semiconductor substrate, focus detecting light such as halogen light is generally projected onto the semiconductor substrate obliquely from above, and the position is detected based on reflected light from the semiconductor substrate. Refer to Japanese Patent Kokai No. 2002-203785 (Patent Document 1).
A semiconductor of silicon carbide (SiC) that is a compound of carbon and silicon has a band gap of 3.25 eV, which is three times as wide as that of the Si semiconductor. Hence, its field intensity at which to reach breakdown is 3 MV/cm and about ten times as high as that of the Si semiconductor. Further, the SiC semiconductor is characterized in that it is high in saturated drift velocity, is excellent in thermal conductivity, heat resistance, and chemical resistance, and is higher in the resistance to radiation than the Si semiconductor. These characteristics enable the production of power devices and high-frequency devices which are much smaller, lower in loss, and more efficient than ones made of the Si semiconductor and of semiconductor devices excellent in radiation resistance. Accordingly, there are large needs for SiC devices in astronautic and nuclear fields as well as electric power, transport, electric appliance fields. Recently, the study of their use as semiconductor devices for use in hybrid vehicles has gained momentum because the advantage has been attracting attention that their power consumption is small and the upper temperature limit is 400° C. and higher than that of the Si semiconductor, thus not needing a dissipating device such as a cooling fan.
However, because SiC is a transparent material having transparency to light, a problem may occur in the process of forming a photo-resist pattern on a SiC substrate. Namely, if substrate material is transparent or semitransparent like SiC, focus detecting light projected by the exposure apparatus to detect the position in the thickness direction of a substrate is transmitted into the substrate and reflected inside the substrate or by the bottom of the substrate, and thus the position in the thickness direction of the substrate is detected based on the reflected light. In this case, the focus detecting light is not always reflected by the same plane, which results in lower accuracy in detecting the position in the thickness direction of the substrate. Thus, positioning control of the wafer stage in the substrate-thickness direction, that is, the focus control of exposure light cannot be correctly executed either. Hence, it is difficult to transfer the pattern of a master mask to a resist with fidelity, and there is a possibility that a resist pattern of a desired shape and dimensions cannot be formed.
Even where the position of a substrate is not correctly detected by the exposure apparatus, for example, if the focus detecting light were always reflected by the same plane throughout a wafer, a desired resist pattern could be formed by setting an offset in the exposure apparatus. However, because the position in the thickness direction of a substrate detected by the exposure apparatus is subject to variation in the refractive indexes of the substrate and the resist, the above method that sets an offset cannot solve this problem with substrates like SiC substrates having many defects therein, namely, having a refractive index varying locally. Thus, if the conventional photolithography process is applied as it is to form a resist pattern on a transparent or semitransparent substrate like a SiC substrate, the focusing of exposure light will be uneven for each exposure, resulting in variation in the shape and dimensions of the finished resist pattern within the same wafer surface and the same lot, and among lots. In a worst-case scenario, the resist pattern itself may not be formed due to an inappropriate exposure process.